1. Field of the Invention
The present invention relates to hydrophobic precipitated silicas of high pH and low silanol group density, to a process for preparing them and to their use, such as in defoamers, fillers or carriers.
2. Description of the Related Art
Hydrophobic precipitated silicas and processes for preparing them are known. Hydrophobicization is accomplished generally by populating the surface of a hydrophilic precipitated silica with suitable organic compounds. Examples of such hydrophobic or partly hydrophobic precipitated silicas are disclosed in patents EP 0 798 348, U.S. Pat. No. 4,377,493 and EP 1 281 735, EP 1 281 733 and EP 1 281 735 disclose hydrophobic precipitated silicas with a pH of 5-9, while WO 2003014020 discloses those having a pH of more than 9.5.
The use of hydrophilic and hydrophobic precipitated silicas in defoamer formulations is likewise known (Pigments Technical Bulletin 42, DEGUSSA, June 1986). Utility in defoamer formulations imposes exacting requirements on the precipitated silicas. Thus they ought to be readily and effectively dispersible into the defoamer formulation and ought to lead to a rapid knockdown time (response time), complete knockdown (immediate effect) and long holddown (service life). Knockdown describes the ability of the defoamer to reduce the height of the foam immediately following addition, down to a defined height of the foam. Holddown characterizes the service life of the defoamer, i.e., the duration of its activity. Specifically a measurement is made of the time taken for the foam to regain a defined level. Alongside these the knockdown time characterizes the time taken to reach knockdown, relative to the foam maximum. Prior art precipitated silicas have unsatisfactory values for some if not all of the stated parameters.